arXiv subjects

Da-Ming Chen

Publications and source records attributed to Da-Ming Chen.

At least 19 recordsLinked to original sources

arXiv2025

Statistical Strong Lensing as a Test of Conformal Gravity

As an alternative gravitational theory to General Relativity (GR), Conformal Gravity (CG) can be verified through astronomical observations. Currently, Mannheim and Kazanas have provided vacuum solutions for cosmological and local gravitational systems, and these solutions may resolve the dark matter and dark energy issues encountered in GR, making them particularly valuable. For static, spherically symmetric systems, CG predicts an additional linear potential generated by luminous matter in addition to the conventional Newtonian potential. This extra potential is expected to account for the observations of galaxies and galaxy clusters without the need of dark matter. It is characterized by the parameter $\gamma^*$, which corresponds to the linear potential generated by the unit of the solar mass, and it is thus a universal constant. The value of $\gamma^\ast$ was determined by fitting the rotation curve data of spiral galaxies. These predictions of CG should also be verified by the observations of strong gravitational lensing. In this study, building upon the previous research, we tested CG via strong lensing statistics. We used a well-defined sample that consisted of both galaxies and galaxy clusters. This allowed us to test CG through statistical strong lensing in a way similar to the conventional approach in GR. As anticipated, our results were consistent with previous studies, namely that the fitted $\gamma^*$ is much larger than that from rotation curves. Intriguingly, we further discovered that, in order to fit the strong lensing data of another sample, the value of $\gamma^*$ cannot be a constant, as is required in CG. Instead, we derived a formula for $\gamma^*$ as a function of the stellar mass $M_*$ of the galaxies or galaxy clusters. It was found that $\gamma^*$ decreases as $M_*$ increases.

astro-ph.CO↗

arXiv2025

Test of conformal gravity as an alternative to dark matter from the observations of elliptical galaxies

As an alternative gravitational theory to General Relativity (GR), the Conformal Gravity (CG) has recently been successfully verified by observations of Type Ia supernovae (SN Ia) and the rotation curves of spiral galaxies. The observations of galaxies only pertain to the non-relativistic form of gravity. In this context, within the framework of the Newtonian theory of gravity (the non-relativistic form of GR), dark matter is postulated to account for the observations. On the other hand, the non-relativistic form of CG predicts an additional potential: besides the Newtonian potential, there is a so-called linear potential term, characterized by the parameter $\gamma^*$, as an alternative to dark matter in Newtonian gravity. To test CG in its non-relativistic form, much work has been done by fitting the predictions to the observations of circular velocity (rotation curves) for spiral galaxies. In this paper, we test CG with the observations from elliptical galaxies. Instead of the circular velocities for spiral galaxies, we use the velocity dispersion for elliptical galaxies. By replacing the Newtonian potential with that predicted by non-relativistic form of CG in Hamiltonian, we directly extend the Jeans equation derived in Newtonian theory to that for CG. By comparing the results derived from the ellipticals with that from spirals, we find that the extra potential predicted by CG is not sufficient to account for the observations of ellipticals. Furthermore, we discover a strong correlation between $\gamma^*$ and the stellar mass $M^*$ in dwarf spheroidal galaxies. This finding implies that the variation in $\gamma^*$ violates a fundamental prediction of Conformal Gravity (CG), which posits that $\gamma^*$ should be a universal constant.

astro-ph.GA↗

arXiv2024

Quantum Effects on Cosmic Scales as an Alternative to Dark Matter and Dark Energy

The spin-torsion theory is a gauge theory approach to gravity that expands upon Einstein's general relativity (GR) by incorporating the spin of microparticles. In this study, we further develop the spin-torsion theory to examine spherically symmetric and static gravitational systems that involve free-falling macroscopic particles. We posit that the quantum spin of macroscopic matter becomes noteworthy at cosmic scales. We further assume that the Dirac spinor and Dirac equation adequately capture all essential physical characteristics of the particles and their associated processes. A crucial aspect of our approach involves substituting the constant mass in the Dirac equation with a scale function, allowing us to establish a connection between quantum effects and the scale of gravitational systems. This mechanism ensures that the quantum effect of macroscopic matter is scale-dependent and diminishes locally, a phenomenon not observed in microparticles. For any given matter density distribution, our theory predicts an additional quantum term, the quantum potential energy (QPE), within the mass expression. The QPE induces time dilation and distance contraction, and thus mimics a gravitational well. When applied to cosmology, the QPE serves as a counterpart to the cosmological constant introduced by Einstein to balance gravity in his static cosmological model. The QPE also offers a plausible explanation for the origin of Hubble redshift (traditionally attributed to the universe's expansion). The predicted luminosity distance--redshift relation aligns remarkably well with SNe Ia data from the cosmological sample of SNe Ia. In the context of galaxies, the QPE functions as the equivalent of dark matter. The predicted circular velocities align well with rotation curve data from the SPARC (Spitzer Photometry and Accurate Rotation Curves database) sample.

gr-qc↗

arXiv2022

Torsion fields generated by the quantum effects of macro-bodies

We generalize Einstein's General Relativity (GR) by assuming that all matter (including macro-objects) has quantum effects. An appropriate theory to fulfill this task is Gauge Theory Gravity (GTG) developed by the Cambridge group. GTG is a ``spin-torsion" theory, according to which, gravitational effects are described by a pair of gauge fields defined over a flat Minkowski background spacetime. The matter content is completely described by the Dirac spinor field, and the quantum effects of matter are identified as the spin tensor derived from the spinor field. The existence of the spin of matter results in the torsion field defined over spacetime. Torsion field plays the role of Bohmian quantum potential which turns out to be a kind of repulsive force as opposed to the gravitational potential which is attractive. The equivalence principle remains and essential in this theory so that GR is relegated to a locally approximate theory wherein the quantum effects (torsion) are negligible. As a toy model, we assume that the macro matter content can be described by the covariant Dirac equation and apply this theory to the simplest radially symmetric and static gravitational systems. Consequently, by virtue of the cosmological principle, we are led to a static universe model in which the Hubble redshifts arise from the torsion fields.

gr-qc↗

arXiv2020

Comparison of Modeling SPARC spiral galaxies' rotation curves: halo models vs MOND

We investigate a sub-sample of the rotation curves consisting of 45 HSB non-bulgy spiral galaxies selected from SPARC (Spitzer Photometry and Accurate Rotation Curves) database by using two dark halo models (NFW and Burkert) and MOdified Newtonian Dynamics (MOND) theory. Among these three models, the core-dominated Burkert halo model provides a better description of the observed data ($χ_ν^2$ = 0.33) than Navarro, Frenk and White (NFW, $χ_ν^2$= 0.45) and MOND model ($χ_ν^2$ = 0.58). So our results show that, for dark halo models, the selected 45 HSB non-bulgy spiral galaxies prefer a cored density profile to the cuspy one (NFW); We also positively find that there is a correlation between $ρ_0$ and $r_0$ in Burkert model. For MOND fits, when we take $a_0$ as a free parameter, there is no obvious correlation between $a_0$ and disk central surface brightness at 3.6 $μm$ of these HSB spiral galaxies, which is in line with the basic assumption of MOND that $a_0$ should be a universal constant. Interestingly, our fittings gives $a_0$ an average value of $(0.74 \pm 0.45) \times 10^{- 8}\rm {cm\ s^{- 2}}$ if we exclude the three highest values in the sample, which is smaller than the standard value ($1.21 \times 10^{-8}\rm {cm\ s^{- 2}}$).

astro-ph.GA↗

arXiv2017

The total density profile of DM halos fitted from strong lensing

In cosmological N-body simulations, the baryon effects on the cold dark matter (CDM) halos can be used to solve the small scale problems in $Λ$CDM cosmology, such as cusp-core problem and missing satellites problem. It turns out that the resultant total density profiles (baryons plus CDM), for halos with mass ranges from dwarf galaxies to galaxy clusters, can match the observations of the rotation curves better than NFW profile. In our previous work, however, we found that such density profiles fail to match the most recent strong gravitational lensing observations. In this paper, we do the converse: we fit the most recent strong lensing observations with the predicted lensing probabilities based on the so-called $(α,β,γ)$ double power-law profile, and use the best-fit parameters ($α=3.04, β=1.39, γ=1.88$) to calculate the rotation curves. We find that, at outer parts for a typical galaxy, the rotation curve calculated with our fitted density profile is much lower than observations and those based on simulations, including the NFW profile. This again verifies and strengthen the conclusions in our previous works: in $Λ$CDM paradigm, it is difficult to reconcile the contradictions between the observations for rotation curves and strong gravitational lensing.

astro-ph.GA↗

arXiv2017

Baryon effects on the dark matter halos constrained from strong gravitational lensing

Simulations are expected to be the powerful tool to investigate the baryon effects on dark matter (DM) halos. Recent high resolution, cosmological hydrodynamic simulations (\citealt{Cintio14}, DC14) predict that the inner density profiles of DM halos depend systematically on the ratio of stellar to DM mass ($M_{\ast}/M_{\rm halo}$) which is thought to be able to provide good fits to the observed rotation curves of galaxies. The DC14 profile is fitted from the simulations which are confined to $M_{\rm halo}\le 10^{12}M_{\sun}$, in order to investigate the physical processes that may affect all halos, we extrapolate it to much larger halo mass, including that of galaxy clusters. The inner slope of DC14 profile is flat for low halo mass, it approaches 1 when the halo mass increases towards $10^{12}M_{\sun}$ and decreases rapidly after that mass. We use DC14 profile for lenses and find that it predicts too few lenses compared with the most recent strong lensing observations SQLS (\citealt{Inada12}). We also calculate the strong lensing probabilities for a simulated density profile which continues the halo mass from the mass end of DC14 ($\sim 10^{12}M_{\sun}$) to the mass that covers the galaxy clusters (\citealt{Schaller15}, Schaller15), and find that this Schaller15 model predict too many lenses compared with other models and SQLS observations. Interestingly, Schaller15 profile has no core, however, like DC14, the rotation curves of the simulated halos are in excellent agreement with observational data. Furthermore, we show that the standard two-population model SIS+NFW cannot match the most recent SQLS observations for large image separations.

astro-ph.GA↗

arXiv2010

Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem

Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter dominated low surface brightness (LSB) galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simulations predict a divergent (cuspy) density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statistics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. This places contradictory demands upon the effects of baryons on the central mass profiles of galaxies.

astro-ph↗

arXiv2009

Cusp-core problem and strong gravitational lensing

Cosmological numerical simulations of galaxy formation have led to the cuspy density profile of a pure cold dark matter halo toward the center, which is in sharp contradiction with the observations of the rotation curves of cold dark matter-dominated dwarf and low surface brightness disk galaxies, with the latter tending to favor mass profiles with a flat central core. Many efforts have been devoted to resolve this cusp-core problem in recent years, among them, baryon-cold dark matter interactions are considered to be the main physical mechanisms erasing the cold dark matter (CDM) cusp into a flat core in the centers of all CDM halos. Clearly, baryon-cold dark matter interactions are not customized only for CDM-dominated disk galaxies, but for all types, including giant ellipticals. We first fit the most recent high resolution observations of rotation curves with the Burkert profile, then use the constrained core size-halo mass relation to calculate the lensing frequency, and compare the predicted results with strong lensing observations. Unfortunately, it turns out that the core size constrained from rotation curves of disk galaxies cannot be extrapolated to giant ellipticals. We conclude that, in the standard cosmological paradigm, baryon-cold dark matter interactions are not universal mechanisms for galaxy formation, and therefore, they cannot be true solutions to the cusp-core problem.

astro-ph.CO↗

arXiv2009

$f(R)$ gravity theories in the Palatini Formalism constrained from strong lensing

$f(R)$ gravity, capable of driving the late-time acceleration of the universe, is emerging as a promising alternative to dark energy. Various $f(R)$ gravity models have been intensively tested against probes of the expansion history, including type Ia supernovae (SNIa), the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO). In this paper we propose to use the statistical lens sample from Sloan Digital Sky Survey Quasar Lens Search Data Release 3 (SQLS DR3) to constrain $f(R)$ gravity models. This sample can probe the expansion history up to $z\sim2.2$, higher than what probed by current SNIa and BAO data. We adopt a typical parameterization of the form $f(R)=R-αH^2_0(-\frac{R}{H^2_0})^β$ with $α$ and $β$ constants. For $β=0$ ($Λ$CDM), we obtain the best-fit value of the parameter $α=-4.193$, for which the 95% confidence interval that is [-4.633, -3.754]. This best-fit value of $α$ corresponds to the matter density parameter $Ω_{m0}=0.301$, consistent with constraints from other probes. Allowing $β$ to be free, the best-fit parameters are $(α, β)=(-3.777, 0.06195)$. Consequently, we give $Ω_{m0}=0.285$ and the deceleration parameter $q_0=-0.544$. At the 95% confidence level, $α$ and $β$ are constrained to [-4.67, -2.89] and [-0.078, 0.202] respectively. Clearly, given the currently limited sample size, we can only constrain $β$ within the accuracy of $Δβ\sim 0.1$ and thus can not distinguish between $Λ$CDM and $f(R)$ gravity with high significance, and actually, the former lies in the 68% confidence contour. We expect that the extension of the SQLS DR3 lens sample to the SDSS DR5 and SDSS-II will make constraints on the model more stringent.

astro-ph↗

arXiv2008

Strong lensing probability in TeVeS theory

We recalculate the strong lensing probability as a function of the image separation in TeVeS (tensor-vector-scalar) cosmology, which is a relativistic version of MOND (MOdified Newtonian Dynamics). The lens is modeled by the Hernquist profile. We assume an open cosmology with $Ω_b=0.04$ and $Ω_Λ=0.5$ and three different kinds of interpolating functions. Two different galaxy stellar mass functions (GSMF) are adopted: PHJ (Panter-Heavens-Jimenez, 2004) determined from SDSS data release one and Fontana (Fontana et al., 2006) from GOODS-MUSIC catalog. We compare our results with both the predicted probabilities for lenses by Singular Isothermal Sphere (SIS) galaxy halos in LCDM (lambda cold dark matter) with Schechter-fit velocity function, and the observational results of the well defined combined sample of Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). It turns out that the interpolating function $μ(x)=x/(1+x)$ combined with Fontana GSMF matches the results from CLASS/JVAS quite well.

astro-ph↗

arXiv2006

Strong lensing probability for testing TeVeS theory

We calculate the strong lensing probability as a function of the image-separation $Δθ$ in TeVeS (tensor-vector-scalar) cosmology, which is a relativistic version of MOND (MOdified Newtonian Dynamics). The lens, often an elliptical galaxy, is modeled by the Hernquist profile. We assume a flat cosmology with $Ω_b=1-Ω_Λ=0.04$ and the simplest interpolating function $μ(x)={\rm min}(1,x)$. For comparison, we recalculated the probabilities for lenses by Singular Isothermal Sphere (SIS) galaxy halos in LCDM with Schechter-fit velocity function. The amplification bias is calculated based on the magnification of the second bright image rather than the total of the two brighter images. Our calculations show that the Hernquist model predicts insufficient but acceptable probabilities in flat TeVeS cosmology compared with the results of the well defined combined sample of Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS); at the same time, it predicts higher probabilities than SIS model in LCDM at small image separations.

astro-ph↗

arXiv2005

Nonsingular density profiles of dark matter halos and Strong gravitational lensing

We use the statistics of strong gravitational lenses to investigate whether mass profiles with a flat density core are supported. The probability for lensing by halos modeled by a nonsingular truncated isothermal sphere (NTIS) with image separations greater than a certain value (ranging from zero to ten arcseconds) is calculated. NTIS is an analytical model for the postcollapse equilibrium structure of virialized objects derived by Shapiro, Iliev & Raga. This profile has a soft core and matches quite well with the mass profiles of dark matter-dominated dwarf galaxies deduced from their observed rotation curves. It also agrees well with the NFW (Navarro-Frenk-White) profile at all radii outside of a few NTIS core radii. Unfortunately, comparing the results with those for singular lensing halos (NFW and SIS+NFW) and strong lensing observations, the probabilities for lensing by NTIS halos are far too low. As this result is valid for any other nonsingular density profiles (with a large core radius), we conclude that nonsingular density profiles (with a large core radius) for CDM halos are ruled out by statistics of strong gravitational lenses.

astro-ph↗

arXiv2004

Flux-limited strong gravitational lensing and dark energy

In the standard flat cosmological constant ($Λ$) cold dark matter (CDM) cosmology, a model of two populations of lens halos for strong gravitational lensing can reproduce the results of the Jodrell-Bank VLA Astrometric Survey (JVAS) and the Cosmic Lens All-Sky Survey (CLASS) radio survey. In such a model, lensing probabilities are sensitive to three parameters: the concentration parameter $c_1$, the cooling mass scale $M_\mathrm{c}$ and the value of the CDM power spectrum normalization parameter $σ_8$. The value ranges of these parameters are constrained by various observations. However, we found that predicted lensing probabilities are also quite sensitive to the flux density (brightness) ratio $q_{\mathrm{r}}$ of the multiple lensing images, which has been, in fact, a very important selection criterion of a sample in any lensing survey experiments. We re-examine the above mentioned model by considering the flux ratio and galactic central Super Massive Black Holes (SMBHs), in flat, low-density cosmological models with different cosmic equations of state $ω$, and find that the predicted lensing probabilities without considering $q_{\mathrm{r}}$ are over-estimated. A low value of $q_\mathrm{r}$ can be compensated by raising the cooling mass scale $M_\mathrm{c}$ in fitting the predicted lensing probabilities to JVAS/CLASS observations. In order to determine the cosmic equation of state $ω$, the uncertainty in $M_\mathrm{c}$ must be resolved. The effects of SMBHs cannot be detected by strong gravitational lensing method when $q_{\mathrm{r}}\leq 10$.

astro-ph↗

arXiv2004

The amplitude of mass fluctuations and mass density of the Universe constrained by strong gravitational lensing

We investigate the linear amplitude of mass fluctuations in the universe, $σ_8$, and the present mass density parameter of the Universe, $Ω_\mathrm{m}$, from the statistical strong gravitational lensing. We use the two populations of lens halos model with fixed cooling mass scale $M_\mathrm{c}=3\times 10^{13}h^{-1}M_{\sun}$ to match the observed lensing probabilities, and leave $σ_8$ or $Ω_\mathrm{m}$ as a free parameter to be constrained by data. Another varying parameter is the equation of state of dark energy $ω$, and its typical values of -1, -2/3, -1/2 and -1/3 are investigated. We find that $σ_8$ is degenerate with $Ω_\mathrm{m}$ in a way similar to that suggested by present day cluster abundance as well as cosmic shear lensing measurements: $σ_8Ω_\mathrm{m}^{0.6}\approx 0.33$ (Bahcall & Bode\cite{bahcall03a} and references therein). However, both $σ_8\leq 0.7$ and $Ω_\mathrm{m}\leq 0.2$ can be safely ruled out, the best value is when $σ_8=1.0$, $Ω_\mathrm{m}=0.3$ and $ω=-1$. This result is different from that obtained by Bahcall & Bode (\cite{bahcall03a}), who gives $σ_8 =0.98\pm 0.1$ and $Ω_m =0.17\pm 0.05$. For $σ_8=1.0$, higher value of $Ω_\mathrm{m}=0.35$ requires $ω=-2/3$ and $Ω_\mathrm{m}=0.40$ requires $ω=-1/2$.

astro-ph↗

arXiv2003

Strong gravitational lensing and galactic bulges

Lensing probabilities of quasars with image separations greater than Δθand flux density ratios less than q_{\mathrm{r}} are calculated by foreground dark matter halos in a flat, cosmological constant dominated (\LambdaCDM) universe . The mass density of the lenses is taken to be the Navarro-Frenk-White (NFW) profile on all mass scales, plus a central point mass for low-mass halos with M<M_c=5\times 10^{13}h^{-1}M_{\odot}. We introduce a quantity M_{\mathrm{eff}}, which is a point mass ranging from 1 to 1000 times the mass M_{\bullet} of a supermassive black hole (SMBH) inhabiting the center of each galaxy, to describe the contributions of galactic central SMBHs and galactic bulges to lensing probabilities. The lensing cross section and thus the lensing probability are quite sensitive to the flux density ratio q_{\mathrm{r}} of multiple images in our calculations. It is shown that, to reproduce the lensing survey results of JVAS/CLASS for q_{\mathrm{r}}<10, about 20% of the bulge mass is needed as a point mass for each galaxy. Since there is still considerable uncertainty regarding the value of the spectrum normalization parameter σ_8, we investigate the effect of varying this parameter within its entire observational range (from 0.7 to 1.1), and find that low σ_8 values (\leq 0.7) are ruled out, and the best fit value is σ_8\simeq 1.0.

astro-ph↗

arXiv2002

Strong gravitational lensing: why no central black holes?

We investigate how central black holes (BHs) inhabited in galactic dark halos could affect strong gravitational lensing. The distribution of integral lensing probability with image separations are calculated for quasars of redshift 1.5 by foreground dark matter halos. The mass density of dark halos is taken to be the Navarro-Frenk-White (NFW) profile such that, when the mass of a halo is less than $10^{14} M_{\sun}$, its central black holes or a bulge is included as a point mass. The relationship between the masses $M_{\bullet}$ of supermassive black holes and the total gravitational mass $M_{\mathrm{DM}}$ of their host galaxy is adopted from the most recent literature. Only a flat $Λ$CDM model is considered here. It is shown that, while a single black hole for each galaxy contributes considerable but not sufficient lensing probabilities at small image separations compared with those without black holes, the bulges (which are about 100--1000 times larger in mass than a typical black hole) would definitely contribute enough probabilities at small image separations, although it gives too high probabilities at large separation angles compared with lensing observations.

astro-ph↗

arXiv2001

Is the X-ray variable source in M82 due to gravitational lensing ?

We explore the possibility of attributing the recent discovery of the hard X-ray variable source CXOM82 J095550.2+694047 in M82 to the gravitational magnification by an intervening stellar obeject along the line of sight as microlens. The duration of the event ($>84$ days) allows us to set robust constraints on the mass and location of the mircolensing object when combined with the dynamical properties of the Galactic halo, the M82 and typical globular clusters. Except for the extremely low probablity, the microlensing magnification by MACHO in either the Galactic halo or the M82 halo is able to explain the X-ray variability of CXOM82 J095550.2+694047. It is hoped that the lensing hypothesis can be soon tested by the light curve measurement.

astro-ph↗